1. Field of the Invention
The present invention relates to a focused ion beam apparatus irradiating a sample with a thinned ion beam and performing a microfabrication processing on the sample, in particular to a focused ion beam apparatus having a column valve.
2. Description of the Related Art
As a focused ion beam apparatus that irradiates a sample with a thinned ion beam and that performs a microfabrication processing on the sample, there is conventionally known an apparatus disclosed in, for example, Japanese Patent Application Laid-Open No. 10-162769.
In the focused ion beam apparatus disclosed therein, processing accuracy (accuracy in finished shape) depends on a thinness of an ion beam, i.e., a magnitude of a beam spot on the sample, and a throughput (processing speed) depends on a current amount of the beam. To perform high accuracy and high throughput processing, it is necessary to use an ion beam as thin as possible and to use high electric current.
The focused ion beam apparatus includes an aperture device configured to include openings (apertures) of various sizes so as to be able to produce beams of various current amounts and to be able to locate an aperture of a desired size onto a central axis of an optical system. Namely, a higher-current beam can be obtained by employing a larger aperture. However, a charged-particle-beam-applied apparatus has the following characteristics. If a larger aperture is employed, a beam spot becomes larger because of lens aberration of the optical system that focuses a charged particle beam on the sample. As a result, it is disadvantageously impossible to process the sample with high accuracy. On the other hand, if an aperture having a smaller diameter with low aberration is employed, a beam spot can be made smaller. However, beam current is lower, so that the throughput is disadvantageously deteriorated.
Considering these disadvantages, the sample is processed by executing a plurality of steps using a high-current beam or a microbeam depending upon circumstances so as to process the sample with as high accuracy as possible and as high throughput as possible. First, an entire processing region of the sample is processed (subjected to rough processing) with low accuracy by a high-current beam obtained by using the larger aperture. A region near a boundary of the processing region is processed (subjected to intermediate processing) by a beam having a beam spot of an intermediate size and an intermediate amount of current. Further, a narrow region on the boundary is finished (subjected to finishing processing) with high accuracy by a microbeam obtained by using the smaller aperture.
Examples of ion beams used in the processings in these steps include an ion beam for rough processing with a current amount of 30 nA and a beam spot diameter of one μm obtained by using an aperture at a diameter of 650 μm, an ion beam for intermediate processing with a current amount of six nA and a beam spot diameter of 0.15 μm obtained by using an aperture at a diameter of 300 μm, and an ion beam for finishing processing with a current amount of 0.1 nA and a beam spot diameter of 0.02 μm obtained by using an aperture at a diameter of 40 μm.
Moreover, the focused ion beam apparatus normally includes a column valve arranged between an ion gun and a sample chamber. The column valve is a valve for evacuation. In a state in which the column valve is closed, even if the sample chamber is in an atmospheric pressure environment, the ion gun can be kept to have a degree of vacuum of about 10−6 Pa. If the focused ion beam apparatus is left for a long time or the sample is replaced by another sample, the column valve is closed for safety against unexpected vacuum leakage.
However, as long as emission from an ion source provided in the focused ion beam apparatus continues, the aperture is constantly irradiated with the ion beam. Due to this, the ion beam passed through one of the apertures is restricted and cut off. Since the ion beam is heavier by 103 to 104 than an electron beam, the ion beam has a sputtering action. When members constituting the apertures are subjected to sputtering by irradiation of the ion beam, surfaces of the members and sidewalls of openings of the apertures are shaved. The surfaces of the members and the sidewalls of openings of the apertures become thinner and the openings of the apertures become wider. As a result, a diameter of each aperture increases, and a desired current amount and a desired diameter of the beam cannot be obtained. This disadvantageously deteriorates a performance of the focused ion beam apparatus as the charged-particle beam irradiation apparatus. To bring the apparatus back into good performance, replacement of the apertures is necessary. Namely, a service life of an aperture ends when the replacement time has comes. The service life of the aperture generally depends on an irradiation amount (irradiation current density×irradiation time) of the beam accumulated per unit area.
An index of a beam performance for evaluating high processing accuracy and high throughput in microfabrication using the ion beam is a current density of the beam. Assuming ion beams having the same beam spot diameter for comparison, the higher-current beam is irradiated when the current density of the beam is higher. Therefore, the sample can be processed at higher speed with the same processing accuracy. In the above-stated example of the ion beams, current densities of the beam for rough processing, the beam for intermediate processing, and the beam for finishing processing on the beam spot are 3.8 A/cm2, 34 A/cm2, and 31.8 A/cm2, respectively. The current density of the beam for rough processing is far lower than those of the beam for intermediate processing and the beam for finishing processing.
If a TEM sample is to be produced using a focused ion beam, it takes time to finish the sample into a thin film. It is, therefore, desired to make the current density of the finishing beam further higher.
Development of an ion-beam-applied microfabrication processing apparatus capable of generating an ion beam at high current density in an entire range of the beam used for microfabrication processing is underway, and the current density of the beam is increasingly higher. Due to this, the ion beam irradiated onto the aperture of the aperture device is concentrated on a narrow irradiation region and the current density of the beam becomes higher. An irradiation amount (irradiation current density×irradiation time) of the beam accumulated per unit area is an indication for determining whether a service life of each aperture expires. A time required until such an irradiation amount reaches an upper limit (time until an irradiated region is thinner and an opening diameter of the aperture begins to change) is disadvantageously shortened.
Moreover, in the focused ion beam apparatus including the column valve, if the ion beam is emitted from the ion source when the column valve is closed, then the column valve is irradiated with the beam, and sputtering particles and secondary electrons are generated. The sputtering particles and secondary electrons collide against a surface wall of a vacuum container in which the column valve is disposed. Absorbable molecules such as hydrocarbons on the surface wall are decomposed and solidified, and deposited to form a high resistance or insulating film. In this way, contaminants disadvantageously increase. To prevent the disadvantage, a high-voltage power supply may be turned off when the column valve is closed. It is thereby possible to prevent emission of the ion beam. However, if the high-voltage power supply is turned on again, it takes some time until emission is stabilized. Due to this, it disadvantageously takes long time to restart the apparatus.